Ultrasonic transducers are used in a wide variety of applications wherein it is desirable to view the interior of an object noninvasively. For example, in medical applications, without making incisions or other breaks in the skin, much diagnostic information may be obtained from an ultrasonic image of the interior of a human body. Thus, ultrasonic imaging equipment, including ultrasonic probes and associated image processing equipment, has found widespread medical use.
However, the human body is not acoustically homogeneous. Depending upon which structures of the human body are serving as an acoustic transmission medium and which structures are the targets to be imaged, different frequencies of operation of an ultrasonic probe device may be desirable.
Current ultrasonic probes include a transducer or a transducer array which is optimized for use at one particular frequency. When differing applications require the use of different ultrasonic frequencies, a user typically selects a probe which operates at or near a desired frequency from a collection of different probes. Thus, a variety of probes, each having a different operating frequency, is often required with acoustic imaging equipment currently in use, adding to the complexity of use and the cost of the equipment.
Prior art dual frequency ultrasonic transducers utilize a transducer with a relatively broad resonance peak. Desired frequencies are selected by filtering. Current commercially available dual frequency transducers have limited bandwidth ratios, such as 2.0/2.5 MHz or 2.7/3.5 MHz. Graded frequency ultrasonic sensors that compensate for frequency downshifting in the body are disclosed in U.S. Pat. No. 5,025,790, issued Jun. 25, 1991 to Dias.
Probes currently in use, such as mentioned above, typically include an impedance matching layer. This layer matches the acoustic impedance of the transducer or transducer array to the acoustic impedance of an object under examination, such as a human body. However, impedance matching layers currently in use are frequency selective. That is, they correctly match the transducer impedance to the impedance of the object under examination only over a narrow band of frequencies. Therefore, current impedance matching layers act as filters, further limiting the usable bandwidth of a probe.